The present invention relates to a method for manufacturing a high purity graphite material and an apparatus for manufacturing the high purity graphite material by using the method.
Hithereto, graphitization of a carbon product has been carried out in the following manner. That is, a generally known method comprises:
a process A in which a carbon product is first heated up to 800.degree. to 1000.degree. C. in a baking furnace, then is baked while dispersing and evaporating volatile components included in a binder and others;
a process B in which the baked material is taken out and put in a graphitizing furnace such as Acheson furnace, Castner furnace or induction heating furnace to be heated up to 3000.degree. C., thereby achieving graphitization of the material; and
a process C in which the material thus graphitized is further heated in a reactor under a gaseous atmosphere which includes halogen, so that impurities included in the graphitized material is transformed to a volatile material and volatilized out of the base material, thereby achieving the high purification.
In the aforementioned conventional method for graphitization and high purification, it is necessary for the carbon material to be delivered from one furnace to the other for each process of baking, graphitization and high purification, which takes much time and labor and moreover there is a possibility of breaking down the material.
It is further required to raise and decrease the temperature for each of the furnaces, which is not economical in view of utilization of heat energy and rate of operation of the apparatus.
Concerning the high purification process, as is disclosed in Japanese Laid-Open Patent Publication (unexamined) No. 58-84181 for example, a method was proposed in which the high purification process is further divided into three steps, i.e., a first step wherein impurities are chlorinated by putting it contact with HCl gas under normal pressure, a second step wherein the temperature of the carbon material is raised under reduced pressure so that metallic chloride in the internal part of the carbon material is easy to desorb, and a third step wherein residual impurities are eliminated by using H.sub.2 gas under nornal pressure after being transformed to a hydride. According to this method, the carbon material to be highly purified is placed in a silica tube and heated from outside the tube.
Another method for manufacturing a high purity graphite was disclosed in Japanese Patent Publication (examined) No. 35-5737, in which the Acheson furnace is adopted and not only chlorine but also fluorinated hydro-carbon can be used under normal pressure.
However, in any of the conventional high purification methods, purification indeed can be attained to a certain extent, but purity of the graphite obtained in this manner is not satisfactory in view of the recent tendency of increasingly demanding a higher purity graphite. Further, in the case of the aforementioned high purification process disclosed in the Publication 58-84181, three steps are separately required, which is not satisfactory in view of pratical use on the commercial basis.
Concerning the type of furnace, Acheson furnace, Castner furnace and induction heating system are conventionally known. In the former two furnaces which are installed horizontally, a problem exists in that a relatively large area is needed and that power efficiency thereof is inferior to the latter since the surface area of the former is larger than the latter.
Recently, there was proposed a relatively advanced method in which after carrying out the baking process A in a separate furnace, the graphitization process B and the high purification process C are then carried out continuously in an Acheson furnace under normal pressure. However, there still exist the following disadvantages in this method. That is, in such method a large area is needed as discussed before, power consumption is also large, and contact between halogen gas and carbon material is not satisfactory due to structural restriction of the furnace. Moreover, since the high purification takes place under the normal pressure, halogenated impurities are not sufficiently desorbed, thereby requiring a rather long operation time resulting in large amount of halogen consumption.
There was proposed a yet further system in which a vacuum type furnace incorporating a heating element of resistor type such as electrode is used. This system is not satisfactory, either, because unevenness in heating is prone to occur and besides heat insulation of components located in the vicinity of a graphite heater (hereinafter referred to as "susceptor") is rather low, which means that this method is not suitable for manufacturing a very high purity graphite material, either. Particularly when it is attempted to shorten and perfectly carry out the halogenating process by repeating fluctuation in the furnace pressure like pulsation as described later, it is almost impossible to carry out operation control. Even if possible, the operation will be considerably complicated. As the result, leakage of halogen out of the susceptor will occur bringing about a further problem of corrosion of metallic materials in the container, connection part of the heater, etc.